Method for monitoring operational condition of circuit components

ABSTRACT

A method and apparatus for enabling the continuous monitoring of the operational state of circuit components includes a scheme for generating a plurality of voltage signals representing neutral-to-line voltages of the system and adding to each of those signals a second signal, each second signal being identical. This addition is done vectorially to yield a modified signal. A representation of the modified signal as acted upon by the circuit to be monitored is compared with a representation of the neutral-to-line voltage signal to yield an output signal representative of the operational state of the circuit components.

This is a divisional of copending application Ser. No. 07/411,977 filedon Sept. 25, 1989, now U.S. Pat. No. 4,996,519.

BACKGROUND OF THE INVENTION

The present invention relates generally to monitoring electrical circuitcomponents and more particularly to a method and apparatus forcontinuously monitoring the operational condition of electrical circuitswhich may provide the same output in response to a normal operatingcondition and to a failed circuit condition.

In certain instances, electrical circuitry such as control circuitry mayprovide the same output in response to a normal operating condition anda failure condition. As an example, in certain amplifying controlcircuitry there may be a zero output in response to a zero input,indicating that there is no extant error condition, and the same zerocondition may also exist if the circuit were to fail. In suchsituations, it is often difficult to determine that this circuit orcomponent is functionally operative. It is possible, of course, toperiodically apply a test signal and to check that the output is of anormal value. This method, however, has two basic problems. The first ofthese problems is that if the circuit is in a direct feedback loop, itmust be disconnected from that loop in order not to disturb the systembeing controlled. The second major drawback is that this type of anarrangement does not provide for the continuous monitoring of thecircuit or component.

As an example in which the above situation exists, attention is calledto the U.S. Pat. No. 4,855,644 "Method and Apparatus for DampingOscillations of AC Generator" by L.J. Lane, issued Aug. 8, 1989. Asrecognized in that patent, the speed of a given generator can oscillateabove or below its synchronous speed such that its period of oscillationis in the approximate range of 0.2 to 2.0 seconds. It is known that thegenerator can effect very little damping to electromechanicaloscillations which can result in spontaneous oscillations or even lossof synchronism with the power system and the attendant large voltagedifferentials. Mechanical oscillation and loss of synchronism may affectthe operational life of the generator and the reliability of the powersystem. This patent describes a polyphase system in which each phaseincludes circuitry, including an amplifier, for controlling thegenerator field excitation. Since the generator may remain in aquiescent state not requiring any change in generator excitation througha power system stabilizer, it is seen that the output of the circuitrycould remain at zero for long periods of time. The concern with thissystem is that because the generator may remain in the quiescent statefor comparatively long periods of time with no change in output signal,there is no assurance that the circuitry is continuously operable.

SUMMARY OF THE INVENTION

It is, therefore, an object of the present invention to provide a methodand apparatus for enabling the monitoring of the operational status ofcertain electrical circuitry.

It is a further object to provide a method and apparatus for enablingthe monitoring of the operational state of circuits and circuitcomponents employed in an electrical system operating on line-to-linevoltages.

It is a still further object to provide a method and apparatus forcontinuously monitoring the operational state of an electrical circuit,especially an amplifier circuit.

An additional object is to provide a method and apparatus for monitoringthe operational state of an amplifier employed in a polyphase system forstabilizing a generator.

The foregoing and other objects are achieved, in accordance with thepresent invention, by a scheme which enables the monitoring of circuitcomponents in a polyphase electrical system operating on line-to-linevoltages through the generation of a plurality of voltage signalsrespectively representing neutral-to-line voltages of said system andvectorially adding to each of said voltage signals an identical signal(a transmission signal) whereby there results a corresponding pluralityof modified neutral-to-line voltage signals without modification ofline-to-line voltages. The present invention further provides for themonitoring of the operational state of an amplifier by providing aninput signal comprised of two components to the amplifier and thencomparing the output of that amplifier with an additional signal whichrepresents one of the two components of the input to the amplifier whichhas been operated upon by a gain corresponding to the gain of theamplifier. In a preferred usage, the amplifier and its monitoring systemare employed in a polyphase system for stabilizing a generator to assurethat certain circuitry for the generator is operational at all times.

BRIEF DESCRIPTION OF THE DRAWING

While the present invention is defined in particularity in the claimsannexed to and forming a part of this specification, a betterunderstanding thereof can be had by reference to the followingdescription taken in conjunction with the accompanying drawing in which:

FIG. 1 is a block diagram of a three phase power system in accordancewith the prior art;

FIG. 2 is a schematic diagram of a portion of FIG. 1, in accordance withthe prior art, which illustrates a preferred environment for employmentof the method and apparatus of the present invention;

FIGS. 3, 4 and 5 are phasor diagrams illustrating a principal employedin the present invention;

FIG. 6 is a block diagram illustrating an aspect of the presentinvention in a first embodiment;

FIG. 7 is a block diagram illustrating an aspect of the presentinvention in a second embodiment; and,

FIG. 8 is a schematic diagram corresponding to FIG. 2 illustrating thepresent invention employed in a three phase application.

DETAILED DESCRIPTION

Before beginning a description of the invention, it is believedappropriate to describe a prior art system in which the presentinvention is believed to have particular application. In this regard,reference is first made to FIG. 1 which is a major block diagram of thisenvironmental system. Previous reference has been made to U.S. Pat. No.4,855,644 which patent is specifically incorporated herein by reference.Those familiar with that patent will recognize FIG. 1 (as well as FIG.2) as being taken therefrom. Specifically with reference now to FIG. 1,there is shown an AC synchronous generator 10 connected to a powersystem illustrated by lines 12. A potential transformer system,illustrated as a wye connected three-phase transformer 14, providessignals proportional to the generator terminal voltage to a power systemstabilizer (PSS) 16 of the type known in the art. The voltage signalsfrom the transformer 14 are also applied to a modulator 20, to which thepresent invention is directly applicable, by way of lines A, B, and C.The secondaries of the potential transformer 14 have their common pointconnected to ground.

Signals proportional to the output current of the generator 10 arederived by suitable current transformers 18. These signals are alsoprovided to the power system stabilizer 16.

The power system stabilizer 16, in response to the input signalsrepresenting generator terminal voltage provides, on line 22, anoscillation signal which has an instantaneous value proportional tooscillations of the generator 10. This signal, via line 22, is appliedto the modulator 20 such that the modulator output, on lines D, E and F(a control signal), will be the signals on lines A, B, and C as modifiedby the modulator 20 in accordance with the signal on line 22. Signalsfrom the modulator are applied to a voltage regulator 24, of a customarytype, the output of which is applied to an exciter 26 which, in turn,controls the excitation of the generator field winding 28.

The function of the modulator in this invention is to add to or subtractfrom (modulate) the generator terminal voltage signals on lines A, B,and C using the power system stabilizer output on line 22 so that thegenerator excitation current becomes modified in accordance with thatlatter signal.

FIG. 2 illustrates the modulator 20 as it was presented in oneembodiment of the aforementioned patent and further includes, as pointsof reference, the three input lines A, B, and C as well as the powersystem stabilizer 16 and its inputs. (The depiction of FIG. 2 is one ofseveral implementations shown in the aforementioned patent and serves,as would all of the embodiments, as a suitable environment for theemployment of the present invention.) Included within the modulator 20is a transformer, indicated generally at 30, which includes a wyeconnected primary winding having primary windings P_(I), P₂ and P₃connected, respectively, to the three input lines A, B, and C. Thecommon point (n') remains equal to the voltage at the neutral of theapplied voltages. Three secondary windings S₁, S₂, and S₃ are connectedin wye configuration with their common point being ground or neutral. Atertiary winding 32, connected in delta, is provided for the flow of thethird harmonic currents to help establish the neutral point.

In accordance with standard transformer theory, the voltages across thesecondary windings S₁, S₂ and S₃, with respect to neutral or ground, asseen respectively at nodes 34, 36 and 38, will be equal to the voltageson lines A, B, and C (with respect to neutral) times the ratio of theturns in the secondary to the primary windings. The three voltagesignals at nodes 34, 36 and 38 are respectively applied as the "y"inputs to three multipliers M₁, M₂ and M₃. The second input (x) to eachof the multipliers is the output of the power system stabilizer 16 online 22. Each of the multipliers M₁, M₂ and M₃ is preferably a fourquadrant multiplier which means that the outputs (z), as seen at therespective nodes 40, 42 and 44, are signals having magnitudes which arethe products of the multiplier inputs and signs according to therelative signs of the inputs.

The signal at the node 40 is applied to a linear amplifier 46 which maybe comprised, in the manner known in the art, of an operationalamplifier 48 having input resistor 50 and a feedback resistor 52connected between its output and its input. The output of the linearamplifier 46 is connected to one end of the primary winding 54 of thetransformer 56 the other end of which is connected to ground.Transformer 56 further has a secondary 58 connected between the A inputand the D output of modulator 20. The voltage induced into the secondarywinding 58 will be proportional to the output of the multiplier M₁, asadjusted by the proportionality factor of the linear amplifier 46, andthe turns ratio of the transformer. Since the secondary winding 58 is inseries with the line from input A to output D, the voltage at the outputD, with respect to neutral, will be equal to the algebraic sum of thevoltage at A and the voltage induced into the secondary winding 58.

In a similar manner, the voltage at node 42 is applied to a linearamplifier 60 the output of which is applied to a transformer 62 inassociation with input B and output E. The voltage signal at node 44 isapplied to a linear amplifier 64 whose output is supplied to atransformer 66 associated with input C and output F. As earlierdescribed, with respect to FIG. 1, the signals on line D, E and F areapplied to the voltage regulator 24 which in turn controls exciter 26and hence, the field excitation on the generator in accordance with theteachings of the prior art.

For a more complete understanding of FIGS. 1 and 2 and the overall powersystem stabilizer system, as well as the other embodiments correspondingto that of FIG. 2, reference is again made to the aforementioned U.S.Pat. No. 4,855,644.

In this environment, that circuitry associated with the amplifiers 46,60 and 64 is that of primary concern. The monitoring of this circuit isthe result of the specific embodiment of the present invention. So longas the power system stabilizer output on line 22 is zero, the outputs ofthe multipliers will be zero. With zero inputs to the amplifiers, therewill be no output to facilitate monitoring of the operational state ofthe amplifiers. Since this quiescent state may last for an extendedperiod of time and the proper operational state of the amplifiers iscritical to the functioning of the overall system, there has been anexpressed desire to be able to continuously monitor this circuitry.

The phasor diagrams of FIGS. 3, 4 and 5 illustrate a concept employed inthe present invention. FIG. 3 shows the three neutral-to-line voltages;i.e., n'-a, n'-b and n'-c. FIG. 4 shows, by the dashed line depiction,the corresponding line-to-line voltages a-b, b-c and c-a. If now, asfurther illustrated by FIG. 3, there is added to each of theneutral-to-line voltages the identical vector voltage signal (i.e.,phasors a-d, b-e and c-f) there is no change in the line-to-linevoltage. This is shown in FIG. 4 by the solid line phasors d-e, e-f andf-d. Thus, relating d, e and f to the lines D, E and F in FIG. 2, it isapparent that the additions of the same phasor to the neutral-to-lineinputs at nodes 34, 36 and 38 will have no effect on the line-to-linevoltage signals between lines D, E, and F.

FIG. 5 shows that the angle (α) of the additional phasor is immaterialwith respect to the line-to-line voltage representations. The same canbe shown to be true for other than three-phase systems although,obviously, the three-phase system is the most common. Thus, what islearned from the showing of these phasor diagrams is that, in apolyphase system, the addition of identical signals to representationsof the neutral-to-line voltage signals, while affecting theneutral-to-line signal will not in any way change the line-to-linevoltages representations.

The employment of this concept in its most basic form is illustrated inFIGS. 6 and 7 which illustrate two embodiments of the basic inventiveconcept. Referencing first FIG. 6, two signals are applied to summingjunction 70. These two signals are labeled "transmission" and "basic".With respect to the description of FIGS. 3, 4, and 5, the basic signalis the signal which is desired to be employed; i.e., the neutral-to-linevoltage signals which have been modulated by the PSS output (line 22).The transmission signal is the additional phasor and has aroot-mean-square (rms) value in excess of zero volts. In FIG. 6 both ofthese signals are provided in a positive sense to junction 70, theoutput of which is applied to an amplifier 72 having a prescribed gainG. The output of the amplifier 72 serves as one of the signals to theoverall system, for example, in the FIG. 2 depiction this wouldcorrespond to one of the signals applied to a transformer 56, 62 or 66.The output of amplifier 72 is also applied to a second amplifier 74which has a gain related to the gain of the amplifier 72, in this case1/G. The output of the amplifier 74 is applied to a second summingjunction 76 in a negative sense while the basic signal is applied in thepositive sense. The output of Junction 76 is applied to an absolutevalue circuit 78 the output of which is applied to a filter circuit 79whose output serves as an input to each of two comparators 80 and 82.Comparators 80 and 82 have second inputs which are, respectively,reference signals defining desired maximum and minimum signal levels.

From the description above, if all the circuitry, particularly amplifier72 which is the primary element being monitored, is functioning properlythe output of the absolute value circuit 78 will be equal to themagnitude of the transmission signal, which, in accordance with thepresent invention and the preceding phasor diagrams always has some rmsvalue other than zero. This remains true for all values of the basicsignal including a zero value. If, however, amplifier 72 ceases tofunction and provides a zero output, then the output of the absolutemagnitude circuit 78 will be equal to the value of the basic signal. Assuch, by properly adjusting the values of the maximum and minimumreference signals to the two comparators 80 and 82 as a function ofallowable values of the transmission signal, the circuit can be made toprovide a suitable alarm. This alarm can be an output of either of thetwo comparators on lines 81 and 83.

The FIG. 7 depiction is similar to that of FIG. 6 and differs primarilyin that the output of the amplifier 72 is provided directly, in thepositive sense to the junction 76 while the transmission signal isprovided to an additional amplifier 86. Amplifier 86 has a gain equal tothe gain of amplifier 72 that is a gain of G. In this case, with allcircuits working properly, the output of the absolute value circuit 78will be equal to the absolute magnitude of the transmission signal asamplified by the amplifier 72. In all fundamental aspects, however, theconcept is identical to that of FIG. 6.

FIG. 8 demonstrates the application of the embodiments of either FIG. 6or FIG. 7 to the FIG. 2 type environment. FIG. 8 is identical to FIG. 2with the exception that the signal at node 34, a signal proportional tothe neutral-to-line voltage of the A phase, is applied as an additionalinput to each of the amplifiers 46, 60 and 64. These are thetransmission signals and are shown as being applied respectively thruresistors R₁, R₂ and R₃. Amplifiers 46, 60 and 64 correspond to thesummation block 70 and the gain blocks 72. Blocks 100, 100' and 100", asthe case may be, represent remaining circuitry as shown within thedashed line blocks of FIGS. 6 and 7.

In FIG. 8, so long as the generator is operational, a voltage will existat node 34 and this voltage will be representative of the system voltagewhich, in normal circumstances will remain reasonably constant (eg±10%).This signal at node 34 serves, in the terminology earlier employed asthe "transmission") signal. The earlier denominated "basic" signal is,as appropriate, the output of a one of the multipliers M₁, M₂ or M₃ andis called the modulation signal in the aforementioned patent. Thissignal can, as was earlier described, be of zero value if the output ofthe power system stabilizer 16 is zero, denoting that the generator andthe power system are operating in the quiescent state.

Thus it is seen that there is shown and described a scheme which enablesthe continuous monitoring of a circuit (or component), particularly anamplifier. This scheme, when used in a three-phase system, readilypermits the utilization of the signals representing line-to-linevoltages without affecting those representations. While the inventionhas been shown and described in its preferred embodiments, modificationsthereto will readily occur to those skilled in the art. For examplewhile the basic concept was shown only in the environment of one of theembodiments of the incorporated patent, as earlier stated this concepthas equal applicability to each of the embodiments therein described. Itis not desired, therefore, that the invention be limited to the specificembodiments shown and described and it is intended to cover within theappended claims all such modifications as fall within the true spiritand scope of the invention.

What is claimed is:
 1. A method of monitoring the operational status ofcircuit components in a polyphase electrical system operating online-to-line voltages comprising the steps:a) generating a plurality ofvoltage signals respectively representing neutral-to-line voltages ofsaid system; b) vectorially adding to each of said voltage signals anidentical signal whereby there results a corresponding plurality ofmodified signals representing neutral-to-line voltages but nomodification in line-to-line voltages and voltage representationsthereof; and, c) examining said modified signal to determine theoperational status the circuit components.
 2. A method of monitoringcircuit components in a polyphase electrical system operating online-to-line voltages comprising the steps:a) generating a plurality ofvoltage signals respectively representing neutral-to-line voltages ofsaid system; b) vectorially adding to each of said voltage signals anidentical signal whereby there results a corresponding plurality ofmodified signals representing neutral-to-line voltages but nomodification in line-to-line voltages and voltage representationsthereof and, c) comparing, for each phase of said polyphase system arepresentation of the respective modified signal as acted upon by thecircuit components to be monitored with a representation of anassociated voltage signal to provide an output signal representation ofthe operational condition of the circuit components to be tested.